Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Vehicles can be configured to operate in an autonomous mode in which the vehicle navigates through an environment with little or no input from a driver. Such autonomous vehicles can include one or more sensors that are configured to detect information about the environment in which the vehicle operates. The vehicle and its associated computer-implemented controller use the detected information to navigate through the environment. For example, if the sensor(s) detect that the vehicle is approaching an obstacle, as determined by the computer-implemented controller, the controller adjusts the vehicle's directional controls to cause the vehicle to navigate around the obstacle.
One such sensor is a light detection and ranging (LIDAR) device. A LIDAR actively estimates distances to environmental features while scanning through a scene to assembly a cloud of point positions indicative of the three-dimensional shape of the environmental scene. Individual points are measured by generating a laser pulse and detecting a returning pulse, if any, reflected from an environmental object, and determining the distance to the reflective object according to the time delay between the emitted pulse and the reception of the reflected pulse. The laser, or set of lasers, can be rapidly and repeatedly scanned across a scene to provide continuous real-time information on distances to reflective objects in the scene. Combining the measured distances and the orientation of the laser(s) while measuring each distance allows for associating a three-dimensional position with each returning pulse. A three-dimensional map of points of reflective features is generated based on the returning pulses for the entire scanning zone. The three-dimensional point map thereby indicates positions of reflective objects in the scanned scene.
The angular resolution of a LIDAR system is defined by at least two parameters, the effective solid angle of each emitted light pulse, and the angular separation between each adjacent measurement point. The solid angle defined by each emitted light pulse is influenced by the narrowness of the emitted pulse (e.g., the amount of beam divergence) and also by atmospheric scattering effects, potential diffraction on the environmental reflective surfaces, etc. The angular separation between adjacent measurement points is influenced by the timing budget of the LIDAR system (e.g., the allowable refresh rate for complete scans of the scene), the total solid angle of the scene being scanned. In some systems lenses are employed to partially diverge emitted pulses such that the solid angle of each emitted pulse is comparable to the angular separation between adjacent points. Diverging the emitted pulses creates broader, less precise, individual measurement points, but allows each measurement point to sample a broader angular region of the scene and thereby avoid missing features situated between adjacent measurement points.